This invention relates to transponders and in particular to transponders using antenna-based retro-reflectors, and to positioning/tracking systems utilising such transponders.
A retro-reflector is a device which reflects incident radiation falling upon it back upon its incident path. It is known to use a retro-reflector as the basis for an active or passive transponder; the rationale behind this is the establishment of a communication link by reflecting an interrogating signal back towards its source, thus eliminating the need for a transmitter and its associated equipment at the target.
In the application of radar to terrestrial traffic control there is an increasing need to detect, track and identify craft within the surveillance area. This is of particular relevance to harbour and airport traffic management, particularly considering small vehicles with inherently low radar cross sections (RCS). At present, simple retro-reflectors comprising corner structures are sometimes employed on small boats to increase their RCS. Clearly, a requirement of future transponders employed in these fields are simplicity, light weight, low cost and low power consumption.
Long range identification is traditionally achieved by actively transmitting a response to an interrogating radar. Such a method is employed by the military xe2x80x9cInterrogate Friend or Foexe2x80x9d (IFF) system. However, a disadvantage of this approach is the requirement for a transmitter and signal processing equipment on board the target. Also, the response signal is transmitted omnidirectionally, which requires a transmitted power which is excessively large to establish the communication link and also pollutes the local radio environment.
It is known to use a retro-reflective antenna array in a transponder. Such arrays are based on the Van Atta array first described in U.S. Pat. No. 2,908,002. The Van Atta array comprises a single or multi-dimensional array of antennas in which antenna pairs are joined by transmission lines of equal electrical length. FIG. 1 of the accompanying drawings shows a single-dimensional Van Atta array comprising six dipole antennas 1 to 6. The dipoles are linked in pairs by means of respective transmission lines as follows:
Dipoles 1 and 6 linked by transmission line 7;
Dipoles 2 and 5 linked by transmission line 8; and
Dipoles 3 and 4 linked by transmission line 9.
The transmission lines 7, 8 and 9 are all of equal electrical length, and it can be shown that the result of such linkage is that an electromagnetic wave front A to F incident on the array will be reflected away from the array back along the direction of incidence.
Consider a plane wave incident at an angle xcex8 to the x-axis. If we take points A to F in the incident wave front, energy from point A is received by antenna 1 and re-radiated by antenna 6, energy from point F is absorbed by antenna 6 and re-radiated by antenna 1, and so on for the other antenna pairs. The path lengths AF, BE and CD are all equal and the radiated power from each antenna therefore adds constructively in the direction xcex8 i.e. the reflected wave is in the same direction as the incident wave.
A problem arises in practical use of the Van Atta array because of the requirement that all of the transmission times have to be of the same electrical length. Fabricating a two-dimensional planar array, for example, is next to impossible if the extra complication and expense of crossovers is to be avoided.
In a first aspect of the present invention, this problem is avoided by providing that the length of the transmission lines interconnecting the individual antennas in the array are of a length l given by:
l=Axc2x1nxcex
where:
A is an arbitrary length;
n is zero, or an integer;
xcex is the wavelength of the electromagnetic wave to be retro-reflected.
If n=0 for all transmission lines then this, in effect, defines a Van Atta array since the transmission lines in such an array are all of an equal arbitrary length. Therefore, it is further provided in the present invention that at least some of the transmission lines have a non-zero value of n. It is therefore inherent in the present invention that not all the transmission lines have the same length.
The elongation of any one of the transmission lines 7 to 9 by an amount equal to a multiple of the wavelength of the incoming signal means that the outgoing signal, when it reaches the plane A-F, will have the same phase as it would have had if the corresponding transmission line had not been lengthened. Therefore the power from each antenna still adds constructively in the direction xcex8, and the array continues to operate in a retro-reflective manner.
However, the arrangement is now frequency sensitive because the outgoing signal will only arrive in phase at plane A-F if the extra length of the interconnecting transmission line is a multiple of the incoming wavelength. This might be perceived as a disadvantage but in fact is often actually an advantage because, particularly in security or military applications, it provides an extra, albeit small, degree of security. In practice, it is not a difficulty that a single interrogating frequency has to be used, particularly when it is borne in mind that the antennas themselves are already frequency selective. Furthermore, the device can act as a filter returning to an interrogating transceiver only a single frequency or, in practice, a passband of frequencies.
The use of unequal length transmission lines, as described above, enables two-dimensional retro-reflective antenna arrays to be fabricated onto any of the known planar media, such as microstrip, stripline, or dielectric or optical waveguides but would also enable arrays to be built in conventional cavity waveguides. Even if the array is non-planar, the ability to make some of the transmission lines of different lengths to others considerably eases the physical design problem of interconnecting pairs of antennas in a two dimensional array. In fact, the invention gives almost complete freedom as to the length of the transmission lines which greatly assists in the design process. Clearly, however, there is an upper limit on length above which transmission line losses will become unacceptable. Also large length differences will give rise to differentials in the loss characteristics as between different transmission lines which may degrade operation if carried to excess.
Given the above constraints, the length A can literally be chosen arbitrarily and can even be zero, but a zero value would impose a pointless limitation on the transmission line lengths, namely that they would all have to be equal to a multiple of the wavelength. More sensibly, the length A may be chosen to be either the smallest or the largest convenient physical length, and the remaining lengths will either be the same as the arbitrary length or, where this is not possible due to physical or other constraints, will be greater or smaller than the arbitrary length by an amount equal to an integer multiple of the wavelength. Thus it is possible for any one array to have just one or two transmission lines which are different in length from the rest or, at the other extreme, every single transmission line may have a different length all subject, of course, to the length formula quoted above.
In an embodiment of the invention, means are provided for modulating the retro-reflected signal so that the target may identify itself to the interrogator. Conveniently, said modulation means comprises a modulation device connected in at least some, and preferably all, of the transmission lines in order to modulate the signal passing from one antenna of the array to another via the transmission line. Each modulation device may comprise active or passive circuitry.
The modulation means may be realised by any device which is operable to modulate the signal so that modulation products are produced in the frequency domain. Thus the modulation may comprise switching in the time domain to achieve amplitude modulation or frequency/phase modulation, or the modulation may comprise various forms of splitting the signal, delaying the split components by varying amounts and thence recombining the delayed components to produce a multiplexed signal comprising a summation of the individual components. In all cases, the modulation process gives rise to modulation products in the frequency domain.
Applications for such a transponder include ground radar for vehicle identification and location, avionic landing systems, terrestrial and space communications, local area networks, and terrestrial traffic control such as harbour and airport management where a large number of small vehicles need to be tracked. Other applications include radar measurement and navigation such as satellite systems, remote sensing, road vehicle systems and general maritime applications. For example, the transponder may be attached to a marine or avionic navigation beacon such as a buoy or a lighthouse. If the retro-reflected signal is modulated with a code unique to the target, the target may identify itself to the interrogator. Alternatively the modulation device may include means for frequency modulating the retro-reflected signal in order to mimic doppler shifting to thus make the target appear to be moving when in fact it is not, or vice versa.
In an embodiment of the invention, the modulation device comprises switching means controlled by a modulating signal, said switching means being operable to cyclically switch an incoming signal about a plurality of paths at least some of which contain circuit elements operable to vary a parameter of the signal and thus modulate the signal at the frequency of the modulating signal. For example, the circuit elements may comprise means for altering the amplitude or the phase of the signal travelling along the transmission line, thus producing an amplitude or phase modulated wave.
In an embodiment, in order to realise amplitude modulation, the switching means is operable to connect said incoming signal alternately to a straight-through connection in which the signal passes through the device with its amplitude substantially unchanged and to a load which wholly or partially absorbs the signal. Likewise, in order to realise phase modulation, the switching means is operable to connect said incoming s signal alternately between two paths of different path length. Preferably the difference between the path lengths is half the wavelength of the incoming signal. The switching means preferably comprises semi-conductor switches such as switching diodes.
In an alternative embodiment of the invention, the modulation device comprises splitter means for splitting the incoming signal into at least two paths, a circuit element being connected in each path and operable to vary some parameter of the signal passing along the path. In addition a combiner means is provided for recombining the signals from the individual paths. Preferably, these circuit elements take the form of delay elements, the arrangement being such that the delay times for the different paths are set differently so that the recombined signal is a summation of the original signal passing along the transmission line, delayed by different amounts.
Up to now it has been assumed that, although the modulation means is able to impart information to the retro-reflected signal, this information is essentially fixed by the characteristics of the modulation means and may comprise, for example, identification information which identifies the item to which the transponder is attached. However, in an embodiment of the invention, the modulation means may be such as to carry transient information, as well as, or instead of, the xe2x80x9cfixedxe2x80x9d information referred to above. To this end, the transponder may further comprise a variable input means such as a sensor for generating a signal indicative of some parameter, perhaps relating to the environment local to the transponder, which signal is passed to the modulating means so as to vary the modulation in some way in accordance with the signal. Thus where, for example, the transponder is on a moving object, the sensor could generate a signal indicative of the position of the object so that an interrogating transceiver will receive back from the transponder not only xe2x80x9cfixedxe2x80x9d information identifying the object but also transient information indicating its position. Sensors might also detect such things as local temperature or pressure, or the existence of malfunctioning equipment.
The manner in which the signal from the sensor operates on the modulation will depend upon the type of modulation being used and the nature of the sensor signal itself.
The variable input means may alternatively, or as well as, include some form of manual intervention device. To this end, the variable input means may include an input device such as a keyboard or keypad which could be used to input information which would effect the modulation means so as to vary the modulated signal. In this way, a person in the vicinity of the transponder could send messages via the retro-reflected signal. Such messages could be stored locally in a memory until such time as the interrogating transceiver sweeps the transponder.
In a further embodiment of the invention, amplifier means are provided in at least some, and preferably all, of the transmission lines in order to amplify the signal passing from one antenna of the array to another via the transmission line. Such amplifier means are preferably bi-directionalxe2x80x94i.e. are capable of amplifying signals travelling in either direction.
The amplifier means may be used instead of, or in addition to, the aforementioned modulation means; indeed the amplifier means may itself be modulated to provide both functions in a single unit.
In a further aspect of the invention there is provided a positioning/tracking system for fixed or movable objects, said system comprising at least one modulated transponder, a transmitter for transmitting a signal to illuminate the or each transponder, a receiver for receiving the retro-reflected signal from the transponder or transponders and discriminator means within the receiver for demodulating the modulated signal from the transponder or transponders to enable a wanted reflected signal to be identified.
For example, each transponder may be attached to a fixed or movable object whose position is to be ascertained. Alternatively the multiple spaced modulated transponders may be used to achieve ranging.
A particular realisation of such a system is a vehicle positioning/tracking system, for example for airports, where the object takes the form of a vehicle such as an airport service vehicle or an aircraft which is taxiing or coming in to landxe2x80x94blind aircraft landing system. For this purpose, a modulated retro-reflector is mounted on the vehicle so that its position can be readily ascertained by means of an associated illuminating transmitter and interrogating receiver. The use of modulation greatly improves the ability of the receiver to discriminate the wanted vehicle from the cluttered background since the modulation can provide a unique radar xe2x80x9csignaturexe2x80x9d which can be identified.
In an embodiment, the frequency of the illuminating transmitter is swept to provide range information. The target in this case either comprises or has mounted thereon a modulated retro-reflector, for example of the type described above.
For the establishment of a duplex communications link, a passive transponder (i.e. one in which there is no transmitter) may be required to change its state depending on a stimulus derived from a control signal contained within the illuminating radiation. The microwave retro-reflective antenna is highly suitable for modification to fulfill the requirement for such a two way link. Since the array comprises antennas and transmission lines which efficiently guide the incident microwave radiation into microstrip circuitry (unlike optical corner-type reflectors), this radiation is available to be sampled and processed by additional logic circuitry. The transponder may then respond to incoming control stimuli and change its state of modulation accordingly.
The transponder may be self-powered, for example by the incident radiation, in a similar manner to a conventional radio tag, but the energy thus collected would not be re-radiated as RF but used only for the modest power requirements of the modulation circuitry integral to the passive array. Since high data rates are possible (10 MHz at present), a transponder operated in this mode could have a xe2x80x9cquietxe2x80x9d, or charging period which is long compared to the period of energy consumption where the modulation is activated, yet still convey an information rate of many Kbits/s. The duty cycles necessary will be derived from the power link budgets available in a given system application.